1. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for soft handoff operations in a communication system operating according to Wideband Code Division Multiple Access (WCDMA) principles.
2. Description of the Related Art
The 3rd Generation Partnership Project (3GPP) recently issued a version of its technical specifications commonly referred to as the WCDMA standard. The publication is well known in the art, and describes the operational standard in a WCDMA system. The standard is still evolving at the time of this application for a patent. A copy of the standard may be obtained by contacting 3GPP, 650 Route des Lucoles—Sophia Antipolis, Valbonne—France, or by accessing the World Wide Web at the internet address www.3gpp.org. The latest version of the standard at the time of this application for a patent, and all previously revised publications of the same, are incorporated by reference herein.
Soft handoff operation is well known. A patent issued to Blakeney, II et al, with U.S. Pat. No. 5,640,414, and a patent issued to Gilhousen et al, with U.S. Pat. No. 5,101,501, both assigned to the assignee of the present application for a patent both incorporated by reference herein, provide methods and systems for providing soft handoff in communications in a CDMA communication system. Generally, a mobile station in soft handoff operation receives and combines multiple signals transmitted from different cells in order to improve the overall decoding and demodulation of the information. The signals transmitted from the cells involved in the soft handoff operation carry the same information. The mobile station after decoding and demodulating the signals combines the data metrics in a soft summing operation to add the data energy. As a result, the information data is received with less error.
In CDMA systems, including WCDMA systems, each cell may have a unique pseudo random noise (PN) sequence. Downlink signals transmitted from each cell are multiplied with the PN sequence as a part of the signal spreading function. The downlink signal received by a mobile station is subjected to a de-spreading operation in accordance with the PN sequence used by the transmitter. The result of the de-spreading operations at the data symbol level is combined with the result of de-spreading operation of other signals transmitted by other cells involved in the soft handoff operation. Signals originating from the same cell but arriving at the mobile with different delays due to multipath can be combined in the same manner. A commonly known Rake receiver may be used for the combining operation.
When performing the soft-combining operation, it is important that the mobile combines the matching symbols received from the cells involved in soft handoff. For example, the first symbol of a particular frame from a first cell needs to be combined with the first symbol of the matching frame from a second cell, and so on. The mobile station needs to know the frame timing of the data signals from each cell. Due to factors including the possibility of cell timing being asynchronous with each other, the possible timing offset between the PN spreading code frame timing and the data frame timing, and the uncertainties in the propagation delay, the mobile cannot necessarily rely on the arrival timing of the symbols from each cell to determine which symbols should be combined.
Therefore, there is a need for a method and apparatus for ensuring that a mobile station is aware of the data frame time offset of each downlink signal transmitted by each cell involved in a soft handoff operation at the mobile station.